With development of information equipment, the needs for low-power and thin display apparatuses have grown, so that extensive study and development have been made on display apparatus fitted to these needs. Such display apparatuses includes a display device which permits handwriting input of graphic and character images while effecting pressing with a pen (stylus) or finger without using a keyboard (hereinafter, this function is referred to as a “pen input function”) in view of use in outdoors, power saving and spacing saving. This display device is used in a wearable PC (personal computer), an electronic note pad, etc.
A liquid crystal display device is known as the display device but has been accompanied with problems when it is provided with a pen input function. More specifically, most of liquid crystals have no memory characteristic, so that it is necessary to continuously apply a voltage during display (input of graphic or characters), thus resulting in an increase in power consumption. On the other hand, with respect to liquid crystals having a memory characteristic, it is difficult to ensure reliability on the assumption that the resultant liquid crystal device is used in various environments as in the wearable PC. As a result, it is difficult to commercialize the liquid crystal device.
As another type of a display apparatus having a memory characteristic and of a low-power and thin type, an electrophoretic display apparatus has been proposed by Harold D. Lee et al. (U.S. Pat. No. 3,612,758).
This type of electrophoretic display apparatus includes a pair of substrates disposed with a predetermined spacing, an insulating liquid filled in the spacing between the substrates, a multiplicity of colored charged migration particles (electrophoretic particles) dispersed in the insulating liquid, and an upper electrode (disposed on a viewer side substrate) and a lower electrode (disposed on a rear substrate) which are disposed along the respective substrates at each pixel. The electrophoretic particles are electrically charged positively or negatively, so that they are adsorbed by the upper electrode or the lower electrode depending on a polarity of a voltage applied to these electrodes. As a result, it is possible to display an image by utilizing a state in which the electrophoretic particles are adsorbed by the upper electrode and are observed from a viewer side and a state in which the electrophoretic particles are adsorbed by the lower electrode so that the color of the insulating liquid is visually identified. This type of the electrophoretic display apparatus is referred to as a vertical movement type electrophoretic display apparatus.
On the other hand, Japanese Laid-Open Patent Application (JP-A) No. Hei 9-211499 has disclosed a horizontal movement type electrophoretic display apparatus. This type of electrophoretic display apparatus, different from the vertical movement type electrophoretic display apparatus including the upper and lower electrodes disposed to sandwich the insulating liquid, includes electrodes 13a and 13b which are disposed along one substrate 10b so as to move electrophoretic particle 12 in a direction along the substrate 10b as described in detail later with reference to FIG. 6. The horizontal movement type electrophoretic display apparatus displays an image by utilizing a difference in color between a dispersion state of the electrophoretic particles in a broad area and an accumulation (collection) state of the electrophoretic particles in a narrow area while using a transparent insulating liquid 11.
As a device capable of inputting graphics and characters while applying pressure with a pen or finger. JP-A Hei 5-324163 has proposed a resistance film type coordinate position detection device. By using the electrophoretic display apparatus and such a detection device in combination, it becomes possible to realize a paper like display which, e.g., permits the wearable PC of power and space saving type and can take notes.
In a conventional display device having the pen input function, when pen input is performed, a position coordinate of the pen is detected and written over an image which has already been stored in a display memory. Thereafter, similarly in an ordinary display, data is read from the display memory frame by frame and is sent to a display panel. As a result, the image overwritten with the pen is displayed on a display picture area (screen).
Incidentally, in an ordinary electrophoretic display apparatus having no pen input function, such a driving method wherein a reset drive is performed before effecting an image writing drive has generally been used. More specifically, a display state is once reset to white or black. This is because it is necessary to erase a previously displayed image in order to display a fresh image since the electrophoretic display apparatus has a memory characteristic. The rewriting with resetting includes a case where it is performed by separating reset scanning and writing scanning on a field basis and a case where it is performed by continuously effecting resetting and writing on a line basis.
The electrophoretic display (apparatus) has a relatively slow display response to voltage application when compared with CRTs (cathode-ray tubes) and liquid crystal displays, so that it is used principally for displaying a still image, e.g., in an electronic book or previous display. For these purposes, the entire picture area is ordinarily rewritten on a page basis, so that a resultant image can be viewed with less inconformity even when the picture area is once reset to pure white or solid black.
However, even for the purpose of still image display, at the time of pen-based input, rewriting operation frequently occurs in order to reproduce a trail of a pen as fast as possible. If the frequency of rewriting operation is low, the trail of the pen is displayed late on the picture area even when the pen is moved on the picture area, so that a user feels a considerable inconformity. Accordingly, the display apparatus is required to have a moving image-level rewriting frequency at the time of pen input. However, in the case of performing the above-described driving method using the reset drive and the writing drive, the reset state is visually identified flickeringly. As a result, a display quality is lowered. This flickering is particularly noticeable in the electrophoretic display since it has a low response speed. Even if a so-called partial rewriting operation for scanning only a rewritten portion is performed in order to promote reflectance of the pen input in display, it is no different from the fact that the reset state is visible to eyes.
Further, when the position coordinate of the pen input is overwritten in a display memory, information on the trail of the pen is stored partially, i.e., so-called on a piecemeal basis, in a certain rewriting cycle since the rewriting cycle of the display memory is previously determined. Accordingly, display of the pen trail is also performed on a piecemeal basis, so that a line or a character inputted with the pen is not displayed as the pen is moved. The user also feels inconformity with respect to this phenomenon.
In some cases, such an “undo” function that only an additional image written with a pen or the like is erased so that the display state is returned to a previous state before writing of the additional image is added to the display apparatus, but when a reset drive is performed in such cases, flickering is caused to occur, thus lowering a display quality.